1. Field of the Invention
This disclosure is related to the field of silicone release coating technology. Specifically using catalysts containing bismuth (bismuth (“Bi”) catalysts) for thermal curing of silicone release coatings.
2. Description of Related Art
Release coatings are generally used to prevent things from sticking together. This simple statement and function encompasses a broad base of technology and a large global industry involving both silicone and non-silicone materials. A very common release coating in the industry utilizes thermal curing based generally on the following reaction:

In the above reaction, a high molecular weight silanol prepolymer (such as α,ω-dihydroxysilanol of polydimethylsiloxane (PDMS) structure which has a molecular weight of about 5 kg/mol) reacts with a lower molecular weight silane (such as one with a molecular weight of about 2 kg/mol). The high functionality of the silane provides for the crosslinking of the silanol and the resultant curing of the coating through the formation of an infinite 3D polymeric network. The reaction proceeds slowly at room temperature, but dramatically accelerates in the presence of a catalyst and under elevated temperatures. The reaction is dehydrogenative condensation which is accompanied by evolution of dihydrogen.
The first thermal-cure silicone release coating systems were commercialized in the 1950s and used tin based materials as the catalyst. Since that time, several technological evolutions have occurred including solvent-based platinum (“Pt”)-catalyzed thermal curing systems in the 1970s, followed by solventless platinum (“Pt”)- and rhodium (“Rh”)-catalyzed systems, radiation curing systems and low-temperature curing systems. It should be recognized that terminology in this area can be a bit confusing. When one refers to a platinum (“Pt”) catalyzed curing system, or platinum (“Pt”) catalyst, the catalyst generally does not comprise only platinum metal, instead, it generally means that a compound including that metal is used. This terminology will be used throughout this disclosure, and thus any phrase indicating that the curing system is metal (“Me”)-catalyzed or there is a metal (“Me”) catalyst should be taken to mean that the catalyst is a compound including the metal “Me”, not necessarily the metal itself.
Despite these changes in thermal cure solvent-based release coating systems, tin (“Sn”) catalyst cure systems are still heavily utilized in release coatings. In these reactions, organo-tin compounds, such as dibutyltin diacetate, in the presence of moisture, catalyze the reaction. Tin (“Sn”)-catalyzed condensation cure systems are still generally well used in the art because of their inherent properties. First, the rates of tin (“Sn”)-catalyzed condensation cure systems are slow at room temperatures, becoming faster at higher temperatures. Further, many commonly used manufacturing systems in the art have anchorage and pot life requirements, for which the slow cure times of tin (“Sn”)-catalyzed systems are ideal. In certain cases low-temperature cure is often desirable because it reduces energy costs and facilitates the coating of temperature-sensitive film substrates.
Second, tin (“Sn”)-catalyzed silicone release coating systems are more cost effective. Tin (“Sn”)-catalyzed systems are relatively inexpensive, especially when compared to platinum (“Pt”) or rhodium (“Rh”)-catalyzed systems, which are extremely expensive. Third, tin (“Sn”)-catalyzed silicone release coating systems are extremely robust. For example, tin (“Sn”)-catalyzed systems are resistant to substrate inhibition, thus allowing for a wider choice of possible substrates. Finally, there is generally little adhesive interaction with tin (“Sn”)-catalyzed systems which can be valuable in applications involving silicone laminates. Furthermore, release coatings are usually solvent-borne systems which allow coatings as thin as about 100 nm on a substrate. In sum, tin (“Sn”)-catalyzed silicone release coating systems provide for a well-established, reliable, low-cost coating system that can be coated onto a wide selection of substrates. For this reason, tin (“Sn”)-catalyzed silicone release coating systems are preferred in many areas of the art.
However, despite these advantages, due to environmental concerns and regulatory restrictions there has been pressure to move away from tin (“Sn”)-catalyzed systems. While some in the art have adjusted to these new environmental concerns and regulatory pressure by moving towards platinum (“Pt”)- or rhodium (“Rh”)-catalyzed systems or solventless, emulsion-based and UV-cure technologies, none of these systems have the same inherent advantages of tin (“Sn”)-catalyzed silicone release coating systems. Thus, these alternative systems generally do not meet customer demands for a catalyzed release coating that functions similarly to a tin (“Sn”)-catalyzed release coating. Further, due to increased costs of alternative catalysts, among other factors, these systems are not as cost-effective for producers and manufacturers in the art as the tin (“Sn”)-catalyzed systems.
Because of these cost considerations and behavioral differences in solventless, emulsion-based, platinum (“Pt”)- or rhodium (“Rh”)-catalyzed systems and UV-cure technologies, there is a very high demand in the industry for a catalyst for silicone release coating systems that is cost effective and has the same behavioral characteristics as known for tin (“Sn”)-catalyzed systems. Stated differently, a replacement catalyst is needed that can be used instead of catalysts comprising tin compounds in silicone release coating systems that will retain all of the advantageous properties of tin (“Sn”)-catalyzed systems without any of the toxicity hazards associated with the tin (“Sn”)-catalyzed systems.